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I have this planet in my story that is similar to Earth in many ways. It is inhabited by humans, the flora and fauna are similar, the atmosphere is similar... except not quite.

On Earth, the atmosphere consists of roughly 78% nitrogen, 20% oxygen and 1% other stuff. However, on this other planet, nitrogen takes up a lot less of the atmosphere, coming up in second place with about 30%, with the remaining ~38% being composed of a fictional gas (let's call this Element X) that is about as harmful to living creatures as oxygen and nitrogen are (in other words, completely harmless).

As Element X is a part of the atmosphere, it is also included in the natural production/consumption cycle that, on Earth, simply put, consists of trees converting nitrogen into oxygen and everyone else doing the opposite.

So ignoring Element X, my question is: what would be the effects of the decreased amount of nitrogen? Specifically, I'm referring to the effects it would have on the environment and on the creatures living on the planet, keeping in mind both the planet and its flora/fauna are similar to Earth.

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    $\begingroup$ A few things: plants convert CO² to oxygen, and animals convert it back. Not nitrogen. And oxygen is in fact incredibly harmful to living things, and when plants first started pumping it out it caused a mass extinction; it reacts with pretty much everything. We've now evolved a lot of complicated mechanisms to keep it from reacting with the wrong things and killing us. $\endgroup$ – Draconis Sep 4 '17 at 19:02
  • $\begingroup$ (The main consumers of atmospheric nitrogen are bacteria, rather than plants; plants rely on these bacteria to get the nitrogen compounds they need, and legumes in particular keep colonies of these bacteria in their roots. Animals then get the nitrogen compounds from the plants.) $\endgroup$ – Draconis Sep 4 '17 at 19:06
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    $\begingroup$ "Trees converting nitrogen into oxygen and everyone else doing the opposite": I didn't know trees practiced a particularly advanced form of alchemy... You may want to brush up on the nitrogen cycle, oxygen cycle and carbon cycle. Whether a lower partial pressure of nitrogen would be acceptable depends on the physiology of diazotrophs (nitrogen-fixing bacteria and archaea). $\endgroup$ – AlexP Sep 4 '17 at 19:10
  • $\begingroup$ First it is carbon-dioxide not nitrogen. Second if the flora and fauna evolved in this environment, they probably would not be that similar to Earth; they could be less dependent on nitrogen-compounds in the soil, and in fact if gas X is not completely inert (like xenon) then it will likely bind some things more tightly than nitrogen does; denying those nutrients to plants, so they (and the animals that consume them) must evolve differently. $\endgroup$ – Amadeus Sep 4 '17 at 19:23
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    $\begingroup$ @Lars "Inert" doesn't mean "always a gas"; it means "doesn't react very much". Nitrogen for example is an inert gas compared to oxygen, and helium even more so. All three also have liquid states, but liquid or gas, oxygen is much more likely to react with anything around. $\endgroup$ – Draconis Sep 4 '17 at 21:23
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It depends how X differs from nitrogen; potentially, not much changes

Nitrogen in the atmosphere is effectively inert. It reacts with almost nothing, isn't used by plants or animals…for the most part, it's just sitting there, taking up space.

You say X is harmless, implying that it's also pretty much inert in its gaseous form. (Oxygen, notably, is the opposite; it reacts with all sorts of things, and is incredibly toxic to any life without special protection. We have a word for oxygen reacting with organic matter in an uncontrolled way; it's called "fire".)

Nitrogen, however, has some very reactive compounds (such as ammonia and ammonium). And these compounds are important for plant and animal life on Earth. Historically we've relied on bacteria for this: certain types of bacteria take gaseous nitrogen from the air and turn it into ammonium ions. Plants then take this ammonium from the soil and convert it to various other nitrogen compounds; animals get these by eating the plants. Look into the "nitrogen cycle" for more information on this.

(More recently, humans have found ways to turn nitrogen into reactive compounds on our own. We then call the compounds "fertilizer" and use them on the plants.)

So if X is pretty much nitrogen by a different name, your world might have different bacteria which make reactive X compounds, and plants and animals which use X partially in place of nitrogen. Not much would be different except the name.

If X behaves differently, then we're going to need more information on what exactly is different about it.

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    $\begingroup$ How can X replace nitrogen and nothing would be different? That makes no sense chemically. I believe your answer is already too science based for this question. So if X is pretty much nitrogen by a different name, your world might have different bacteria which make reactive X compounds, and plants and animals which use X partially in place of nitrogen. Not much would be different except the name. this is simply not possible. Of course that is the problem with the premise, but I wouldn't answer until details are given, this might make everything worse $\endgroup$ – Raditz_35 Sep 4 '17 at 19:31
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    $\begingroup$ @Raditz_35 The question specifies that this world contains a fictional gas similar to nitrogen; I'm assuming it's similar in the ways that matter (e.g. even though somehow it isn't actually nitrogen, it forms the same sorts of bonds, has a similar mass...) $\endgroup$ – Draconis Sep 4 '17 at 19:35
  • $\begingroup$ I understand your post, please try to understand mine. This question currently is being downvoted for a reason (not by me), answering very flawed science with very flawed science and telling him everything is ok is imo not a good way to deal with it. If there is this fictional gas, well, fine, I've read and seen worse, but I believe saying that "of course everything is alright, it makes perfect sense" isn't doing any good. My advice is to focus on the nitrogen $\endgroup$ – Raditz_35 Sep 4 '17 at 19:38
  • $\begingroup$ @Raditz_35 Ahh, I see your point. That makes sense. $\endgroup$ – Draconis Sep 4 '17 at 19:41
  • $\begingroup$ Btw I'm sorry I always post too quickly and then have to edit for it to make sense $\endgroup$ – Raditz_35 Sep 4 '17 at 19:41

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